ext: simplelink: Add SimpleLink DPL porting layer to Zephyr

This ports the SimpleLink WiFi host driver via its OS adaptation
layer to Zephyr OS primitives.

This was validated using an out-of-tree SimpleLink shell
application including functions for:
* WLAN connect, disconnect and scan
* Socket: UDP server and client
* NetApp: pinging an external host.

Jira: ZEP-1958

Signed-off-by: Gil Pitney <gil.pitney@linaro.org>